1. Field of the Invention
The present invention relates to a production plant for producing hot-rolled flat products. The production plant includes a rolling train composed of a plurality of roll stands, a run-out table with devices for cooling the hot strip, and with subsequently arranged coiling machines for coiling the strip.
2. Description of the Related Art
Rolling plants for producing hot-rolled strip are generally constructed and operated today in such a way that the deformation in the individual stands takes place austenitically; this means that it is ensured that the rolling temperatures in the individual roll stands are above the GOS line of the iron/carbon diagram. Cooling to reeling temperature with a structure transformation in the cooling stretch or in the wound-up coil takes place only after the last pass which, for obtaining a fine-grained final structure, should be as close as possible to the GOS line.
In the case of low alloy carbon steels, for rolling in accordance with the above-described method, the final rolling temperature, i.e., the temperature in the last roll stand of the rolling train, is about 840.degree. to 920.degree. C., depending on the carbon content. The final rolling temperature is maintained by changing the final rolling speed, wherein the drive power of the roll stands is capable of controlling the natural cooling of the strip in the rolling train and the supply of heat. This method can be used without problems for strip thicknesses above a minimum strip thickness, which, depending on the type of rolling train, is in the order of magnitude of 1.3 mm. If the wall thickness drops below this minimum thickness, the required rolling speed reaches values above 12 meters per second at the strip head; this speed is so high that the strip can no longer be handled at the free runout on the roller table following the rolling train.
In spite of the problems discussed above, the tendency is to develop smaller final thicknesses; this means that the tendency was to reduce the final rolling temperature in order to facilitate slower rolling speeds. These rolling processes have become known under the name ferritic rolling because they take place at temperatures below the GOS line, i.e., in the range of the .alpha., .gamma. mixed crystals or below the GPS line in the ferritic range.
In accordance with a preferred method, a break-down rolling is carried out in the austenitic range to an intermediate thickness of 2 to 8 mm and finish rolling is carried out in the ferritic range to the smallest final thicknesses below 1.3 mm. Between these two process stages, the rolling stock must be cooled from the final rolling temperature in the austenitic range to the rolling temperature in the ferritic range. This means a cooling from the temperature range of 840.degree. C. to 920.degree. C. to the temperature range of 600.degree. C. to 780.degree. C.
The final rolling temperature following the second deforming stage is also in the range of 600.degree. to 780.degree. C. and, thus, in an order of magnitude in which a recrystallization of the structure occurs after reeling in the wound-up coil. A structure is produced which facilitates the use of the product without further cold deformation or heat treatment.
An essential requirement for a good result is that a minimum cooling time is maintained for cooling from the austenitic range into the ferritic range. This minimum cooling time must be maintained in order to make it possible that the transformation from austenite to ferrite can take place to a sufficient extent. Depending on the selected temperature and alloy composition, this minimum cooling time for entering the first ferritic transformation is in the order of magnitude of a few seconds to several minutes.
The realization of the above-described process is very difficult in a conventional rolling train for hot-rolled wide strip. The transformation from the austenitic range into the ferritic range should be carried out in the thickness range of 2 mm to 8 mm, i.e., in a thickness range in which the rolling stock is approximately in the middle of a conventional finishing train. Since the travel time of the strip from one stand to the next stand in the middle of the finishing train takes only a few seconds, it is possible in principle to realize the cooling, however, it is not possible to realize the time required for the transformation. Accordingly, in a conventional rolling train for rolling hot-rolled wide strip, it is not possible to carry out ferritic rolling in accordance with the above-described method.